1. Field of the Invention
This invention relates to an improvement in a semiconductor device, for example a gate-turn-off thyristor having a pair of main electrodes and a control electrode wherein a control signal is applied between one of the paired main electrodes and the control electrode to control conduction between the paired main electrodes.
Especially, the present invention is suitable for obtaining high-power semiconductor devices or in short, power devices adapted to control high power electricity, especially, semiconductor devices adapted to control large current.
2. Description of Prior Art
Semiconductor devices are known which are adapted to control the conduction of large currents by having an arrangement which includes a plurality of emitter regions each surrounded by a control or gate region, or, in short a, semiconductor devices of the multi-emitter type. For example, reference should be made to U.S. Pat. No. 3,611,072 issued to D. R. Hamilton on Oct. 5, 1971. According to the disclosure in this patent, in a gate-turn-off thyristor (hereinafter simply referred to as GTO thyristor) having a disc-shaped semiconductor substrate, cathode regions are surrounded, in a plane of one major surface of the substrate, by a gate region and arranged to extend radially from the center of the disc to the periphery thereof. Each of the cathode regions is in contact with a cathode electrode and the gate region is in contact with a gate electrode. Each cathode electrode is also surrounded, on the plane of the one major surface of the substrate, by the gate electrode. In using the GTO thyristor, all of the cathode electrodes are connected electrically and the entirety is handled as a single cathode electrode.
The arrangement mentioned above is of significance with regard to applications which require turning off large load currents with high efficiency. More specifically, the elongated cathode which is surrounded by the gate electrode is effective to ensure that differences or irregularities in distances between the island of the cathode and the gate can be minimized and, consequently, the turn-off signal from the gate electrode effectively exerts its effect as all of the cathode regions. In addition, a large number of cathode regions cooperate as a whole to enlarge the main current flowing region and hence increase current capacity of the GTO thyristor.
However, in attempting to further increase the current capacity, the prior art faces many difficulties. More particularly, in order to enhance the current capacity with the multi-cathode or generally, multi-emitter arrangement as described so far, such an expedient as increasing the number of those cathode regions or widening the individual separate cathode regions or both of these expedients are required. The excessive enlargement of the area of the individual cathode regions, however, prevents uniform turn-off within the individual cathode regions and therefore this expedient is applicable only within a limited extent. Similar inconveniences are encountered when the length of the cathode region of a strip shape in a plane of one major surface of the substrate is elongated with its width unchanged. Further, the expedient of increasing the number of the cathode regions will require an unduly large space if the cathode regions are radially arranged to be confined within a single concentric zone as in U.S. Pat. No. 3,611,072. Also, in increasing the number of the cathode regions, all of the cathode regions are required to be applied with the turn-off signal with minimal localization or substantially without localization. Therefore, it is necessary to take into account not only the arrangement of the cathode regions but also the geometry of the gate electrode and the manner of applying the turn-off signal to the gate electrode from outside of the device. In U.S. Pat. No. 3,611,072, integral ballast resistors are employed for turn-off uniformity but no advance is found in the arrangement for the gate electrode and cathode regions which is simply a so-called center-gate structure that is well known in the art.